Numerous types of position detection systems are well known in the prior art. Generally, these systems are configured to use techniques employing a single transmitter and multiple receivers, estimating geometric distances by the transmit-receive mode of ranging, to measure the position of a target point. Most ultrasonic systems are of the pulse-echo type and use highly directional transducers that are pointed toward the target point to measure the position and distance of the target point. With this technique, the distance between a transmitter and a receiver is determined indirectly by measuring the elapsed time, usually called the time of flight (TOF), during which the signal generated at the transmitter hits the receiver and bounces back to the transmitter. This ranging technique is used mostly to measure distances in one dimension along the line perpendicular to the transducer surface.
The most popular pulse-echo type system was built by Polaroid which was designed to provide the distance from a camera to the picture subject for the purpose of focusing a lens. However, pulse-echo systems are not very accurate because different reflection properties of the targets affect the accuracy of the TOF. In addition, ultrasonic beams spread as they travel away from the transmitter, thus these systems are unable to effectively point to a small target.
Acoustic ranging systems for applications in two-dimensional and three-dimensional space are produced commercially by a few manufacturers. These products use transducers operating in the transmit-receive mode primarily for digitization applications. The coordinates of a transmitter are calculated from its distance to various receivers located outside the operating volume of the digitizing unit. The distances are determined by measuring the TOF of the ultrasonic pulse as it travels from the transmitter to each receiver. Calculation of the coordinates of the transmitter from these distances is done using a simple triangulation operation.
For example, U.S. Pat. No. 4,991,148 issued to Gilchrist discloses a three-dimensional acoustic digitizing system employing two transmitters and four receivers to determine the position of a target point. A different technique was employed in U.S. Pat. No. 4,862,152 issued to Milner. In Milner, the distance between a transmitter and several receivers were determined using a transmitter responsive receiver frame, a plurality of fixed receivers, and a controller port plug which provides a computer with position signals.
The accuracy of the sonic sensors used in these systems and their response to the changes in the speed of sound has generated much concern. The deterministic effects of the speed of sound in an environment is caused by changes in temperature and humidity, and by air turbulence. For accurate measurements, the speed of sound must be known at all times and the path travelled by the signal must be linear. Because it is difficult to maintain an homogenous environment within a given work area, calibration techniques have been devised. The most common technique employed has been to estimate and to average time of flight measurements.
Gilchrist discloses the use of a reference correction transmitter and a scale correction technique to compensate for the changes in the speed of sound while calibrating the system. One disadvantage of this and similar techniques is that they require a separate transmitter to account for the changes in the speed of sound. Other disadvantages include requiring a controller device to determine the distance between the transmitters and receivers, and a reference receiver with dedicated circuitry to correct for the changes in the speed of sound.
Another technique used to increase the accuracy of the distance of a target point includes calculating the phase shift between the transmitted and received signals. Such a method is disclosed in U.S. Pat. No. 4,752,917, issued to Dechape. Dechape discloses a phase measurement technique that compares the phase of the transmitted signal to the phase of the received signal, and combines this resultant signal with rough time of flight measurements to refine the position of a target point. While Dechape seems to have solved part of the accuracy problem, it does not account for changes in the speed of sound or the speed of sound's deterministic effects on the accuracy of the position of the target point.
Accordingly, what is needed is a detection means and method which provides an inherent self-calibration technique that eliminates the speed of sound as a necessary variable, while determining the coordinates of a point of interest in an m-dimensional coordinate system. The present invention meets these needs.